(a) Field of the Invention
The present invention relates to the device and a method for detecting and locating faults and/or partial discharges in a gas-insulated electrical equipment of the type comprising a closed wall housing containing an insulated gas such as SF.sub.6, for electrically insulating at least one conducting element centrally extending inside the housing.
More particularly, the present invention relates to a device and a method for detecting and locating faults and/or partial discharges in a gas-insulated bus or cable, a gas-insulated substation or a gas-insulated switchgear.
(b) Description of the Prior Art
It is well known in the art that faults and/or partial discharges in a gas-insulated equipment such as a gas-insulated cable or switchgear, generally correspond to malfunction problems that may often lead to failures.
To protect the equipment as much as possible and to reduce the maintainance cost, various devices and methods have already been proposed to detect and locate the faults and partial discharges, in view of eliminating their sources which can be, for example, the presence of an equipment component not bounded to the conductor or to its housing, the presence of free conducting particles in the insulating gas, or a failure in the dielectric material used in the equipment. The faults or discharges which may directly damage the equipment or decompose the insulating gas and create a corrosion problem, generates a high frequency electromagnetic signal which can be detected acoustically. The faults or partial discharges can also be detected by an electromagnetical method such as the one disclosed by S. A. BOGGS et al, IEEE Trans. volume PAS-100, No. 8, August, 1981. This method makes use of a plurality of coaxial capacitive couplers connected to the housing of the equipment by a resistor and an inductor. The couplers which are designed to have a high pass characteristic, detect the electromagnetic pulses which propagate away from a discharge source in opposite direction. By measuring the time interval between the detection of a given pulse by all the couplers, it is possible to locate the faults and/or discharge source, provided that the length between each pair of couplers is known. For this measurement, use can be made, for example, of an integrating autocorrelator such as the one developed by the Research Division of Ontario Hydro (see S. A. BOGGS, IEEE, Trans PAS-101, p. 1935, July 1982).
This known method is quite efficient but has some drawbacks. One of these drawbacks is the position of the couplers which extend around the conducting element inside the equipment housing and disturb the electrical field around this conducting element to such an extent that it may cause in time another source of fault. Another drawback is the presence of unavoidable free particles left over during the construction of the equipment, which may deposit onto the surface of the couplers and cause malfunction thereof.
It has also been suggested in the art to use current sensors for measuring the variation of current in time at a given point of a transmission line. The use of such an current sensor is disclosed, by way of example, by C. A. EKDAHL, Rev. Sci. Instrument. 51, 1645 (1980). The current sensor disclosed by C. A. EKDAHL is used for monitoring a current wave formed in a high density gas-embedded Z pinch, and consists of an annular channel machined in the surface of one flange of a pair of flanges used for attaching to each other sections of the equipment housing. An insulating gap is machined in the flange surface to force the current in the inner wall of the housing to flow around the annular channel. An O-ring can be provided in the gap for preventing the insulating gas from escaping into the annular channel. The voltage generated across the annular channel is picked up by a series resistor connected by an insulated wire between the opposing flanges. In this particular case, the picked-up voltage is not solely proportional to the derivative of the current flowing through the central conductor of the equipment with respect to the time. It also includes the ohmic voltage drop associated with the flow of current on the inside surfaces of the opposing flanges.